The present invention relates to lighting control systems, and in particular, to controls for illumination systems associated with inspection systems. Specifically, the invention provides a power control system that protects a dissipative load, such as an LED illumination array, from reaching life-shortening or destructive temperature levels.
Digital data and signal processing techniques and technology have tremendously advanced the ability to use computers as data processing systems to accomplish sophisticated inspection procedures without human intervention. Almost every type of product can benefit from low cost, high precision, high speed inspection technology derived from these new digital data and signal processing techniques.
For example, in computers and other electronic systems, the electrical connections between electronic components (xe2x80x9cchipsxe2x80x9d) are critical to the operation of the system. As a result of recent technological advances, electronic components are decreasing in size and increasing in complexity, requiring a larger number of electrical connections to be made in a smaller area. Inspection of the electronic components during a manufacturing process helps assure that electrical contacts are properly formed and prevents failed electrical connections between electronic components.
In order to properly inspect such electronic components, sophisticated illumination systems and methods have been developed. One such illumination system, which is especially suitable for illuminating ball grid arrays (BGAs), which are commonly used in manufacturing electronic components, is disclosed, for example, in commonly-owned U.S. Pat. No. 5,943,124, which is fully incorporated herein by reference. t
The ""125 teaches the use of a ring-shaped light source, which includes a plurality of light emitting elements, such as light emitting diodes (LEDs). While this light source is designed especially for use in illuminating BGAs for inspection purposes, various configurations of LED arrays may be employed for a wide variety of illumination sources for a wide variety of inspection applications.
However, one drawback of using LED arrays as illumination sources is that LEDs are dissipative (resistive) loads. Accordingly, as an LED array, or any other dissipative/resistive load for that matter, is powered, it will heat up. If the heat build up is allowed to progress, uncontrolled, the temperature of the array may reach a destructive or life-shortening level.
Various systems and methods have been employed in the past to prevent dissipative/resistive loads from exceeding certain pre-defined life-shortening temperature levels. These systems and methods include the use of basic systems and methods of maintaining a temperature that employ convective cooling, e.g. forcing cool air over the array using a fan or the like.
More sophisticated control systems have been employed as well. One such system controls the temperature of an LED array, thus ensuring that the peak and average temperatures of the array fall within safe limits, by enforcing a maximum pulse width of an LED power signal (during which the LED array is powered) and a minimum off time between pulses. This type of control system employs a simple digital circuit that simply generates a delay after each pulse.
A slightly more sophisticated prior art system computes an inter-pulse minimum delay based on the then-current pulse width. An even more sophisticated prior art system even takes the pulse repetition rate into account.
Since all of the prior art control systems are based on theoretical average thermal characteristics, they do not take into account the real-time, actual heat generation of an LED array. Therefore, a margin of safety must be factored into all prior art control systems. These built-in safety margins necessarily reduce the actual time of array illumination, which in turn limits the throughput of the inspection systems with which they are associated.
Accordingly, it would be advantageous, and a significant improvement over the prior art, to provide a power control circuit suitable for use in controlling dissipative/resistive loads, and in particular, LED illumination arrays, that accurately models the heat being generated by the resistive load that it is controlling. In this manner, arbitrary, built-in safety margins could be eliminated. This would provide a significant improvement in inspection system throughput. It would also make it possible to input a complex series of pulses of varying widths and intervals, such that power to the LED array could be arbitrarily switched without restriction, provided the modeled maximum temperature limit was not exceeded.
The disclosed invention overcomes the drawbacks associated with the prior art control systems by providing a power control system which serves to protect a dissipative/resistive load from exceeding a predetermined temperature limit by accurately modeling the temperature of the load and by enforcing a cooling period to reduce the temperature of the load being protected to a base level if the predetermined temperature limit is reached.
Also provided is a method of selectively connecting and disconnecting a resistive load to and from a power source to prevent the resistive load from exceeding a predetermined high temperature limit. The method begins by registering a voltage proportional to an instantaneous current passing through the resistive load. This is accomplished by wiring a current sensing resistor in series with the resistive load. A load temperature is then modeled using a load temperature analog circuit to produce an output voltage proportional to a current temperature of the resistive load. The output voltage of the analog circuit is then compared to a voltage proportional to a predetermined high temperature limit.
A switch is selectively controlled to disconnect the power source from the resistive load when the high temperature limit is reached and to re-connect the power source to the resistive load when a base temperature is reached.